How can I convert .MOV to .JPEG or .JPG in macOS Sierra 10.12.6 ?

Here are two ways you can get a jpg from a mov file (assuming all you want is to get ONE frame as an image):

1) Use iMovie, import your mov, then park the skimmer over the image you want and do File->Share->Image:

2) Easier (but limited to what size you can display on your screen).

Open the mov file with Quicktime Player, show the image you want, then press command-shift-4; then press space, and click on the QT window. This will save the contents of the window as an image (by default, it will be as png, and saved to your desktop; if you really need jpg, you can then open in Preview and save it as jpg).

Although I am no expert, surely the MOV format contains a whole series of "stills". For example, at say, 25 frames per second, a 5 second movie/video will contain 125 "still" pictures (or frames). If you have a method of capturing any one of these frames, you might be able to convert it to a JPEG image. I am unfamiliar (yes, really!) with the workings of an iPhone so I don't know if you can make a single "still" image of a single frame. I would imagine that it's a whole new ball game to capture, say, all 125 frames as stills, but I imagine this is what you would have to do if you want to convert the whole movie sequence to JPEG format

Although I am no expert, surely the MOV format contains a whole series of "stills".

No, that's not how it works.

You can freeze a frame and export it as Luis mentioned, but it is not a single image file. It has to be extracted from the video stream.

Have a look at this old discussion: .mov to jpeg conversion should be still valid.

Please clarify what you want to achieve.

Do you want to get one frams of video as an image? Or take a whole mov and get it as a an image sequence?